Liquid crystals (LC) consist of anisotropic molecules. The average direction of the long molecular axes is called the director, d. Reorientation of the director caused by the application of an external electric field is the basis of operation of most LC devices. The basic unit of LC devices is a LC cell, which consists of two substrates with LC material sandwiched in between.
The sensitivity of a LC material to an applied electric field is determined by the dielectric anisotropy, Δ∈a, and spontaneous polarization, P. P has a nonzero value only for some chiral smectic LC phases. The higher the Δ∈a and P, the lower are the operating voltage and the faster the electro-optical response of the LC device and thereby, the faster the switching time between light and dark states of the LC cell.
Nematic LC's are the most commonly used LC materials. Their electro-optical response is typically related to the square of the electric field. To increase Δ∈a and P, multi-component LC mixtures have been developed and special molecular dopants have been synthesized. This approach is extremely laborious and very expensive.
Ferroelectric particles are particles which have a spontaneous electric polarization that is reversible by an electric field. It is known that the sensitivity of isotropic liquids to an applied electric field can be increased by doping with ultra-fine (less than 1 micrometer (μm) size) ferroelectric particles. For example, Bachmann and Bärner showed that ferroelectric BaTiO3 particles that have been finely milled in the presence of surfactant will form a stable suspension in heptane (“Stable Suspensions of Ferroelectric BaTiO3-Particles,” Solid State Communications, 68(9), 865-869 (1988)). The particles had an average radius of about 10 nm. The birefringence of the suspension, which is impossible to achieve in a pure heptane matrix, was controlled by application of an electric field.
Müller and Bärner suggested that a radius of approximately 20 nm was the size distribution cut-off for BaTiO3 particles for maintaining a permanent dipole moment, with only smaller particles maintaining the permanent dipole moment (“Polydisperse Suspensions of BaTiO3-Particles,” Ferroelectrics, 108, 83-88 (1990)). More recently, Schurian and Bärner produced stable ferroelectric suspensions of nanometer sized particles of LiNbO3 and PbTiO3 in a hydrocarbon carrier. These suspensions displayed similar birefringence to that of BaTiO3 suspensions. Ferroelectric particles having an average radius of about 10-15 nanometers (nm) were determined to carry a permanent dipole moment of about 2000 Debye (De). Suspensions of nanometer size ferroelectric particles were also created by Schurian et al. by a method which included chemical precipitation of the particles and the use of alternate stabilizers (Journal of Electrostatics, 40 & 41, 205-210 (1997)).
None of these studies however, examined the behavior of ferroelectric particles in a suspension of liquid crystal or other anisotropic material.